Sound recording and reproducing apparatus



March 3, 1931.,

B. E. G. MITTELL Filed Nov. 18, 1925 MAJ fiatentecl Mar. 3, 193i 4 UNITED STATES PArN- r OFFICE BBENGHLE-Y ERNEST enonen I ELL, on IVER, ENGLAND, ASSIGN'OR To vroroia TALKING MACHINE. COMPANY, or CAMDENLNEW JERSEY, A CORPORATION OF'NEW JERSEY SOUND nnoonnnve AND nnrnonnonve arrannrus- Application filed November 18, 1925, Serial No. 69,850, and in Great Britain- December 24, 1924.

This invention relates to apparatus of the v 1 provisions for the recording or reproducing stylus to be able to rise and fall, irrespective of the vibrations of the stylus in following the sound line, owing to the surface of the record, in the case, for example of a disc rec- 0rd, not revolving in an absolutely'true plane.

In known forms of electricalrecording and reproducing apparatus the whole electromagnetic system comprising magnet, pole shoes, coils carrying audio-frequency ourrents, armature and stylus, with their attendant fittings, are counterbalanced and move up and down with the surface of the record.

It is also desirable in apparatus of, this type to employ a relatively strong magnetic field in order to allow for adquate damping of the recording or reproducing stylus and in view of theresistance to be overcome in cutting the wax or with a view to producing sufficiently strong currents to give a'repros duction of desired volume, and this necessitates the provision of a relativelyheavy electro or permanent magnet. If the mass of such magnet has to move up and down to accommodate irregularities in the movement of the surface of the record or blank, even though this mass is counter-balanced, yet it still has a large moment of inertia and superfluous work has to be performed by the recording or reproducing point.

According to the present invention, therefore, the electro-magnetic system is so disposed that certain of its components, prefer ably the heavier member or members, are unable to rise and fall with the surface of the record, whilst the recording or reproducing stylus is capable of up and down movement in addition to its oscillatory movement for recording or reproducing sound waves.

According to one embodiment of the invention, and that which .is at present preferred,

an electro-ma'gneticrecording or reproducing device is constructed in such a manner that the magnetic flux passes through three or more relatively movable parts, the heaviest of which is not required to rise and fall with the surface of the record. Thedevice may conveniently comprise an electro or permanent magnet mounted in such a manner that the magnet poles are not free to rise and fall but (if the recording or reproducing device travels across the record) are capable of movement only parallel tothe surface of the record blank, (or the record may of course travel beneath a fixed recording device), a pair of pole shoes mounted so as to be free to move up and down between said magnet poles, with the surface of the record, and carrying the coils traversed by the audio frequency currents, andan armature, carry ing the'stylus, mounted upon said pole shoes in such a manner as to be free to oscillate between them. We prefer to constructlthe armature in the form of a tube of magnetic material, so that it may be both light and rigid, and provided with a longitudinal slot to hinder the circulation of eddy currents.

For the purpose of description the essen 'tial components of an electromagnetic recording or' reproducing device may be broadly divided into four parts, in the cases in which an armature of magnetic material is employed to carry the stylus, namely: the magnet, which may be permanent or an electromagnet,the pole shoes, the coil which is adapted to carry the audio frequency currents and the vibratable-armature. In the cases in which the stylus is'mounted upon a coil orconductor adapted to vibrate in a magnetic field, the system may be divided into the following parts the magnet, the pole shoes, the vibratable coil or conductor and the magnetic core within the-coil, if such a core is provided. V

In the description which follows the case of a recording device will be discussed and the magnet may thus be considered to be stationary whilst theturntable is rotated and moved horizontally in a straight line beneath the stylus. In the case of a reproducingdevice the magnet may of course be mounted mounted upon the balanced parts.

so as to be rotatable in a horizontal plane above the rotating record and its weight may be taken by the bearing about which it is rotatable. The parts which are so mounted that they are capable of rising and falling with the surface of the record or of the recording wax may be counter-balanced to the desired extent by a movable counterpoise weight and these parts will be referred to as the balanced parts. The member which is adapted to vibrate and which carries the stylus will be referred to as the vibratable member. The vibratable member must of course always be adapted torise and fall.

According to the form of the invention which has already been referred to, the magnet is fixed, the pole-shoes and coils are balanced whilst the armature is vibratably In an other form the magnet is fixed, the pole-shoes are balanced whilst'the vibratable member comprises a coil adapted to carry the audio frequency currents. Alternatively, the mag net and pole-shoes may be fixed and the balanced parts comprise a carrier within which a coil, together with an iron core, is vibratable.

There are a very great number of possible variations within the scope of the present invention and a few of these will now be described, by way of example, in more detail with reference to the accompanying drawings, in which Fig. 1 is a sectional side elevation of one form of recording device.

Fig.'-2 is a front elevation of the same device with an end cover plate removed.

Figs. 3 to 5 are diagrammatic front elevational views of other forms of recording devices.

Referring to Figs. 1 and 2, the permanent horse-shoe magnet 1 is fixed to the base of the recording instrument. The bracket 2 is secured to the magnet 1 and upon the end of the bracket is a pin 8 upon which two members 4, placed upon opposite sides of the bracket 2, are rotatably mounted. A counterpoise arm 5 is fixed to the upper end of the members 4 and carries an adjustable counterpoise weight 6. An end plate f non-mag netic material is fixed to the lower ends of the members 4 and two pole shoes 8 and 9 are attached to the end plate. At the other side the two pole-shoes are connected together by means of an end plate 10, also of nonmagnetic material. The pole-shoes taper towards their inwardly pointed ends so as to concentrate the flux in the air gap. Two coils 11 and 12 are mounted upon a former 13 and rigidly mounted within the framework formed by the end plates 7 and 10 and the pole-shoes 8 and 9. u

The armature 14 is made in the form of a tube, split longitudinally and flattened at its upper end 15. A spindle 16 is rigidly attached to the armature and has its ends adapted to receive the points of pivot screws 17 and 18 which screw through the end plates 7 and 10 respectively. The armature is damped and biased towards a central position by means of two pads 19, of semi-resilient material such as felt carried upon two brackets 20. These pads bear against the two sides of the flattened upper end 15 of the armature 14.

The members 4, the end plates 7 and 10, and the pole-shoes, together with all the parts carried upon these, are mounted so as to be capable of rising and falling between the poles of the magnet 1. The pole-shoes do not touch the magnet poles but the clearance is made as small as possible. The armature 14 carryin the'stylus 21 is capable of vibrating about the pivots 17 and 18.

it will therefore be seen that the magnetic flux is generated in a fixed magnet, passes through the pole-shoes, which are capable of movement between the magnet poles, and through the armature, first in one direction and then in the other as the polarization of the armature is varied by the audio frequency currents and it is only the pole-shoes (which may be made quite light), the coils and the armature, with their necessary fittings, which have to move to accommodate the rise and fall of the surface of the disc. 7

The magnetic flux through the pole-shoes and armature will remain substantially constant for the very small up and down movements which are normally met with in practice. If, however, it is desired to allow for greater movement, the width of the magnet poles may be increasedrelatively to the width of the pole-shoes. I

In the example shown diagrammatically in Fig. 3, the. pole-shoes 8 and 9 are formed integral with the magnet 1. The coils 11 and 12 are carried upon a frame 21 which'may be balanced as shown in Fig. 1. The armature 14 is mounted upon a. spindle 16 which is vibratably mounted in the frame 21. The two brackets 20 carrying damping material 19 are fixed to the frame 21 and the damping material bears upon the ari'nature 14 and gives it a bias towards a central position. In this case also the flux from the magnet 1 passes through three relatively movable parts, the coils 11 and 12 being movable relatively to the fixed magnet 1 and the armature 14 being capable of vibration relatively to the coils and the magnet.

In Fig. 4 the pole-shoes 8 and 9 are formed integral with the magnet 1 and the coils 11 and 12 are fixed within the pole-shoes. The armature 14 is mounted upon a spindle 16 which is carried on pivots upon a suitable balanced frame (not shown). The damping means 19 are fixed to the frame and bear upon the flattened upper end of the armature 14. In this case the heavier members, namely" the magnet 1, the pole-shoes 8 and 9 and the coils 11 and 12, of the electromagnetic system are fixed whilst the armature 14: is capable of up and down movement in addition to its oscillatory movement.

The arrangement according to Fig. 5 is.

similar to that shown in Figs. 1 and 2 excepting that in the present example the stylus 21 is fixed to a vibratable coil 22. The poleshoes 8 and 9 are balanced as in Fig. 1 and support, by means of a bridge piece 23 of non-magnetic material, a cylindrical or spherical core 24. The coil 22 is pivoted upon the core 2 1, so as to be capable of vibration about an axis, perpendicular to the plane of the paper, through the point 25. Damping means 19 are carried upon the pole-shoes and bear upon the sides of the stylus support 25.

In this example the flux from the magnet '1 passes through three relatively movable parts namely the pole-shoes 8 and 9 and the core 24;, which are movable relatively to the magnet 1, and the coil 22 which is capable of vibrating relatively to the pole-shoes and core and relatively to the magnet 1.

The examples given are only a few of the very many possible forms which the invention may take. Other known electromagnetic systems may be used in conjunction with the invention and the mechanical details are also capable of Wide variation. As used in the claims, the term sound record includes an uncut element as well'as one carrying sound grooves. i

I claim; 1. Apparatus electrical vibrations and a sound record through the mediumof a magnetic flux, comprising more than two relatively movable components, one of which is adapted to vibrate in engagement with said record, each of'said remaining relatively movable components forming apart of the path of the magnetic flux which passes through the vibrating component.

2. Apparatus for the inter-conversion of electrical vibrations and a sound record through the medium of a magnetic flux, comprising a plurality of elements forming parts of the magnetic circuit, one of said elements being adapted for movement in one plane only relative to the record, and another of said elements being adapted to engage directly the surface of the record for vibratory movement and being mounted independently of and for bodily movement relative to said element having movement in a single plane.

3. Apparatus for the inter-conversion of electrical vibrations and a sound record through the medium of a magnetic flux, comprising a plurality of components forming parts of the magnetic circuit, one of said components being mounted independently of a second component and said first component being adapted to engage directly the surface for the inter-conversion ofv components is adaptedto engage directly the surface of the record for vibratory movement and to rise and fall bodily with said surface relative to said immovable component.

5. Apparatus as claimed in claim 1, wherein said components comprise a magnet, pole shoes'co-operating with said magnet and a vibrating armature.

6. Apparatus of the class described comprising, in combination with a record, a plurality of elements forming parts of a magnetic circuit, said elements including a magnet mounted so as to be capable of swinging about an axis perpendicular to said record surface, the remaining elements comprising pole-shoes mounted for movement in a plane perpendicular to said record surface and between the poles of said magnet and a vibratory member mounted upon said pole shoes and adapted to vibrate freely between them. 1

7. In apparatus as claimed in claim 6, a stylus carried upon said vibratory member.

8. In apparatus as claimed in claim 1, a stylus carried upon said vibratory component and said vibratory component being formed of magnetic material.

9. In apparatus as claimed in claim 1, a

stylus carried upon said vibratory component and said vibratory component being formed as a longitudinally slotted tube of magnetic material.

10. Apparatus for the inter-conversion of electrical vibrations and a sound record through the medium of a magnetic flux comprising a magnet mounted to swing about an axis perpendicular to the record surface, and a coil and vibrating armature mounted for movement in a plane perpendicular to the record surface between the poles of said magture being freely vibratable with respect to I said poles.

12. Apparatus as claimed in claim 1 wherein one of said components includes a, magnet and the other of szud components lncludes a stylus-receiving device operatively connected to a movable coil.

13. Apparatus asclaimed in claim 4: Wherein the last mentioned component includes a vibratory-armature and damping means associated with said armature.

In testimony whereof I have signed my name to this specification.

BRENCHLEY ERNEST GEORGE MITTELL. 

